The present invention relates to a method of driving a photosensitive device comprising a matrix of photosensitive pixels of the type produced in particular by techniques for depositing semiconductor materials. The invention relates more particularly (but not exclusively) to the driving of such devices used for radiological image detection.
Techniques for the thin-film deposition of semiconductor materials, such as hydrogenated amorphous silicon (a-SiH), on insulating, for example glass, supports that are used to produce matrices of photosensitive pixels that can produce an image from visible or near-visible radiation. To use these matrices for the detection of radiological images, all that is required is to interpose, between the X-radiation and the matrix, a scintillator screen that converts the X-radiation into light radiation within the band of wavelength at which the photosensitive pixels are sensitive.
The photosensitive pixels that form such a matrix generally comprise a photosensitive element associated with an element providing a switch function. The photosensitive pixel is mounted between a row conductor and a column conductor of the matrix. Depending on the requirements, the photosensitive device then comprises a plurality of photosensitive pixels arranged as a matrix or as a linear array.
The photosensitive element usually consists of a diode connected in series with the switch element. The switch element may, for example, be a transistor produced by thin film deposition techniques. These techniques are known as TFT (thin film transistor) techniques. The switch element may also be a switching diode whose “closed” or “on” state corresponds to the bias that drives it into forward conduction and whose “open” or “off” state corresponds to its reverse bias. The two diodes are connected with opposed conduction directions in a configuration called “back-to-back”. Such an arrangement is well known, for example from French patent application 86/14058 (publication No. 2 605 166) which describes a matrix of photosensitive pixels of the type with two diodes in a “back-to-back” configuration, a method of reading the photosensitive pixels and a way of producing such a photosensitive device. The components forming the matrix are produced from an amorphous semiconductor material that produces persistence. This is due to its amorphous structure, which includes a large number of traps, much more than in crystalline materials. These traps are structure defects which extend over the entire band gap. They retain charges generated during image acquisition. The material stores an image corresponding to a given irradiation and restores the charges relating to this image while the following image is being read, or even while several following images are being read. The quality of the images is thereby degraded.
To attempt to obtain a useful image of optimum quality, a correction is applied to the useful image on the basis of what is called an “offset image”, also known as “dark image” that is generally acquired and stored at the start of an operating cycle. This offset image is the image obtained when the photosensitive device is exposed to a signal of zero intensity and corresponds to a kind of background image. The offset image varies according to the electrical state of the components of the photosensitive pixels and to the dispersion in their electrical characteristics. The useful image is that read whenever the photosensitive device has been exposed to a useful signal that corresponds to exposure to X-radiation. It includes the offset image. The correction consists in subtracting the offset image from the useful image.
To produce a useful image or an offset image, an image cycle is performed, that is to say a sequence formed by an image acquisition phase followed by a read phase and then by an erase and reset phase, as explained in patent application FR-A-2 760 585. During the image acquisition phase, the photosensitive pixels are exposed to a signal to be picked up, whether this signal is a maximum illumination signal or the dark signal. During the read phase, a read pulse is applied to the row conductors addressed for reading the amount of charge stored during the image acquisition. During the erase phase, the photosensitive pixels are erased, generally optically by means of a light flash uniformly distributed over all the photosensitive pixels. During the reset phase, the photosensitive pixels are reset to a state in which they are receptive to a new image acquisition. This resetting is performed by temporarily making the switching element, switching diode or transistor, conducting by means of an electrical pulse sent to the row conductors for addressing the matrix.
Hitherto, the photosensitive pixels of a matrix have been reset by generating a single electrical pulse simultaneously for all the photosensitive pixels of the matrix via the row conductors of the matrix. Unfortunately, the photosensitive pixels are not perfect. They are subject to parasitic elements, the modeling of which will be presented later with the aid of FIG. 3. These parasitic elements are essentially capacitive couplings. The rising edge of the electrical pulse sent to the row conductors of the matrix in order to turn on the switching elements injects charges onto column conductors of the matrix. The rising edge that opens the switching element is critical. The charges created on the column conductors by this rising edge can be discharged only via read circuits connected to the column conductors. These charges take a certain amount of time to be discharged and therefore temporarily cause a variation in the voltage on each column conductor. This voltage variation occurs at the moment when the photosensitive pixel is isolated. More precisely, it is the moment when the potential of the common point A between the photodiode and the switching element is fixed. This potential is essential as it is this which changes during the image acquisition phase owing to the effect of the light radiation received by the photodiode. This potential therefore depends on the voltage of the corresponding column conductor at the precise moment when the switching element opens. The presence of this voltage on the column conductor causes an average offset shift and therefore a loss of dynamic range in the useful image.
In addition, the moment when the switching element opens is not strictly identical for all the photo-sensitive pixels. This moment depends on the dispersion in the threshold voltages for each switching element and on the dispersion of the parasitic elements. This results in a dispersion in the offset for each photo-sensitive pixel.
It has been attempted to reduce the voltage on the column conductor, on the one hand by reducing the amplitude of the electrical pulse sent to the row conductors during the reset phase relative to the amplitude of the electrical pulse sent to the row conductors during the read phase and, on the other hand, by extending the rising edge that opens the switching element so that the switching time of the switching element is longer than the time needed to drain away the charges in the column conductors. This palliative remains somewhat unsatisfactory as it improves only few of the problems encountered. In addition, owing to the dissymmetry of the pulses between the reset phase and the read phase, this palliative results in a few offset stability defects.